In order to decrease the probability of passing counterfeit documents such as bank notes, identification cards, checks and the like, it is known to embed a security thread therein. Known security threads have the form of a flat metal band or plastic strip having a rectangular cross-section. Such security threads which are easily visible and can also easily be felt, permit a simple and rapid examination pertaining to the genuineness of the document. Insertion of the security thread into the paper or plastic layer requires, however, a costly process which is mastered by a potential counterfeiter only with difficulty.
In order to further decrease the probability of counterfeiting, and to permit an automatic determination of the presence of a security thread, and consequently the genuineness of a security blank or document, it is known from German Pat. No. 2,205,428 to provide the security thread with microscopically small holes, which, for example, represent a code pattern, which can again be read out with the aid of light rays or rays of particles. Based, however, on the current state of the art of drilling by means of a laser, a code of the aforementioned type is no longer considered a particularly secure feature attesting to the genuineness of the document.
From German Pat. No. 677,711 it is known to admix fibers of a particular shape or consistency to paper, from which bank notes or the like are to be manufactured, which have an unusual cross-section, and which can be differentiated from the fibers of the paper used for bank notes either by the naked eye, by means of a magnifying glass, or by exposure to ultraviolet radiation, where the special fibers fluoresce differently than the fibers of the standard paper in the bank notes.
From U.S. Pat. No. 1,929,828 issued to Schlitz there is known a security blank, including a sheet of fabric, having denomination indicia in the form of a line or sharply defined form of metal within the body of the fabric.
From French Pat. No. 2,107,714 there is known a bank note containing fibers of a fluorescent type, which is irradiated and the genuineness of the document determined from the radiation scattered from the bank note. The cross-section of the security thread may be either round or rectangular.
In pending application Ser. No. 342,065 one of the applicants of the present invention, Baltes, discloses a security blank with enhanced authenticating features, and a method and an apparatus for determining the genuineness of the security blank.